Journal of the Korean Physical Society, Vol. 63, No. 3, August 2013, pp. 672∼675

Structual and Magnetic Properties of MFe2O4 (M=Ni, Mg)Nano Hollow Spheres

Kensuke Konishi,∗ Taiga Sakurai, Yutaro Nagano, Naoto Manabe and Yugo Morimoto

Department of Physics, Graduate School of Science and Engineering, Ehime University, Matsuyama 790-8577, Japan

(Received 1 June 2012)

Sub-micrometer-sized hollow spheres assembled from MFe2O4 (M= Ni, Mg) nanoparticles havebeen prepared by using the template method with a wet chemical process. A superparamagnetic-type blocking process is observed in the zero-field-cooled (ZFC) and the field-cooled (FC) mag-netizations and magnentic susceptibilities. Furthermore, a spin-glass-like behavior due to surfacespin freezing is found in Ni-ferrite nanoparticles. The surface spin freezing temperatures Tf arequite a bit lower than the blocking temperature, and the magnetic field dependence is differentbetween the nanoparticles and the fabricated hollow spheres. The spin-glass nature of the Ni-ferritenanoparticle is demonstrated by the magnetice field dependence of Tf following the well knownAlmeida-Thouless (AT) line. In contrast, the Tf(H) of the hollow Ni-ferrite sphere hints at failureof the Gabay-Toulouse theory as well as the AT theory.

PACS numbers: 75.75.Cd, 75.50.Tt, 75.50.GgKeywords: Hollow spheres, Ferrite, Magnetic propertiesDOI: 10.3938/jkps.63.672

I. INTRODUCTION

The study of nano-sized particles is a growing field ofresearch due to their unique properties and their techno-logical applications in ferrofluids [1], high-density mag-netic recording media [2] and so forth. The magneticproperties of nanoparticles can differ significantly fromthose of their bulk counterparts. A superparamagnetic-type blocking process is observed in the zero-field-cooled(ZFC) and the field-cooled (FC) magnetization. The co-ercivity increases with increasing particle size while thesaturation magnetization decreases [3]. These propertiesoriginate from reduced size and modification of the in-terparticle interactions [4,5]. Three-dimensional randomdistributions of such nanoparticles give rise to superspin-glass ordering at low enough temperatures. A superspin-glass ordering due to interparticle, random, dipole-dipoleinteractions appears in ferromagnetic nanoparticle sys-tems [6, 7]. Such behavior has been observed in two-dimensinal nanomagnet systems. Zhang and Liu showedthat the magnetization memory and aging effects couldalso be observed in interacting two-dimensional nano-magnet systems by using a nonequilibrium Monte Carlomethod [8].

Hollow spheres are of interest in applications includ-ing catalysis, drug delivary, and photonic devices [9,10].Furthermore, interest in magnetic hollow spheres is grow-

∗E-mail: konishi@phys.sci.ehime-u.ac.jp

ing [11]. Sub-micrometer-sized hollow spheres assem-bled from nanoparticles can be used to produce quasi-two-dimensional nanomagnet system. In a system ofrandomly-arranged particles, dipole-dipole interactionsmay be inconflicts and such a coexistence of disorderand frustration is suggestive of a spin-glass-like order-ing. In this paper, aiming at understanding the natureof low-dimensional nanomagnet system, we investigatethe spin-glass-like behavior of hollow spheres aggregatedfrom spinel-ferrite nanoparticles.

II. EXPERIMENTS

Hollow spherical ferrite nanostructures have been pre-pared through a template method [12, 13]. Nanocrys-talline ferrites have been synthesized by using the co-precipitation method. Stoichiometric aqueous solutionsof M(NO3)2·6H2O (M = Mg, Ni), Fe(NO3)3·9H2O andCH3NH2 were mixed under vigorous stirring at 80 ◦Cfor 1 hour. The resulting dispersions were centrifuged at3000 rpm for 25 min, the supernatant solutions were dis-carded, and the particles were resuspended in distilledwater and ethanol in an ultrasonic bath. This processwas repeated several times, and the so-purified powderswere dried in a desiccator. To produce hollow spheres,we added sub-micrometer-sized polystyrene (PS) latexas a template to the beginning mixture. To obtain hol-low spheres, we coated the PS spheres with ferrite nano

-672-

Structual and Magnetic Properties of MFe2O4· · · – Kensuke Konishi et al. -673-

particles calcined for 1 h in air in a furnace at 425 ∼ 450◦C.

The obtained samples were characterized by usinga RINT-1100 diffractometer (Rigaku) to obtain X-raypowder diffraction (XRD) patterns. The size and themorphology of the as-synthesized products were deter-mined by using transmission electron microscopy (TEM)with a JEM-2100FX (JEOL) microscope at the Inte-grated Center for Science of Ehime University. The mag-netic properties were investigated by using a commercialsuperconducting quantum interference device (SQUID)magnetometer (MPMS-XL7, Quantum Design). The DCmagnetic susceptibility χ(T ) was measured in ZFC andFC conditions at 2 – 300 K with magnetic fields, H , of10 ∼ 2000 Oe. The ZFC magnetization curves M(H)were observed by applying fields up to 70 kOe at differ-ent temperatures. The ZFC and the FC hysteresis loopmeasurements were obtained for values of H from 10 to–10 kOe.

III. RESULTS AND DISCUSSION

The XRD pattern shows peaks corresponding to thespinel MFe2O4 pattern (M = Ni: PDF 44-1485, Mg:PDF 17-464). Assuming spherical particles, average sizesof 4.2 nm for Ni ferrite and 3.7 nm for Mg ferrite wereobtained from the X-ray diffraction lines by using theScherrer formula.

The morphology and the structures of the productswere investigated by using TEM. Figure 1 shows a repre-sentative TEM image of the products with 500-nm PS la-tex as a template. They appear to have a quasi-sphericalshape and a uniform size [Fig. 1(a)], and the mean di-ameters of hollow the spheres are estimated to be 470nm for Mg ferrite and 600 nm for Ni ferrite. The thick-nesses of the hollow spherical shells are about 12 - 17nm. Figure 1(c) clearly shows that the spherical particleshave rough surfaces before calcining. After heat treat-ment, they become hollow spheres composed of manysmall nanoparticles [Figs. 1(a), (b), (d)]. This demon-strates that the small nanoparticles have self-assembledinto aggregated hollow spherical particles. The sizes ofthese small nanoparticles are 5.1 ± 0.4 nm and 3.2 ± 0.4nm for Ni and Mg ferrite, respectively. The size of thenanoparticles from XRD and TEM are very close.

Figure 2 shows a comparison of the magnetizationcurves for bulk, nanoparticle and hollow sphere sam-ples at 3 K after zero-field cooling. For the nanoparticleand the hollow sphere sample, the saturation magneti-zation Ms is estimated using a Langevin profile L(x) ≡coth(x) − 1/x, where x =

µH

kBT, µ is the mean particle

moment, and kB is the Boltzmann constant. The satura-tion magnetization is smaller than the bulks counterpart,the values of Ms for the NiFe2O4 nanoparticle and hol-low sphere samples are 35 emu/g (∼ 66% of bulk; 53

Fig. 1. TEM images of ferrite hollow spheres. (a) Mgferrite, (b) Mg ferrite [magnified], (c) Ni ferrite [as prepared],and (d)Ni ferrite [after heat treatment]

emu/g) and 12 emu/g (∼ 23%) , respectively. A similartrend is also seen for MgFe2O4, for which the values ofMs for the nanoparticle and the hollow sphere samplesare 18 emu/g (∼ 53% of bulk; 34 emu/g) and 9 emu/g(∼ 26%), respectively.

The coercivity obtained from the ZFC and the FC hys-teresis loop measurements increases at the temperaturesbelow the surface spin freezing temperature Tf . In theNiFe2O4 hollow sphere sample, an increase of coercivityis observed at 30 K (>Tf ).

Figure 3 shows the temperature dependence of thedc magnetic susceptibility obtained under ZFC and FCconditions (H = 100 Oe). A typical superparamagneticblocking process appears below the temperature signaledby the onset of irreversibility. Splitting between the ZFCcurve and the FC curve is observed in the susceptibility,and the ZFC curve shows an obvious maximum. In theMgFe2O4 case, the temperature of the maximum posi-tion of the ZFC susceptibility is close to the bifurca-tion temperature. The sizes of the MgFe2O4 nanopar-ticles seems to have an almost monodisperse size dis-tribution both in dispersed nanoparticles and in hollow

-674- Journal of the Korean Physical Society, Vol. 63, No. 3, August 2013

Fig. 2. Field dependences of the magnetizations for thebulk (�), nanoparticles (◦), and hollow spheres (♦) measuredat 3 K: (a) NiFe2O4, (b) MgFe2O4.

Fig. 3. (Color online)Temperature dependences of the ZFCand the FC magnetic susceptibilities for hollow spheres undera field 100 Oe: (a) NiFe2O4 and (b) MgFe2O4.

spheres. On the other hand, in case of the NiFe2O4 hol-low sphere, the temperature of the maximum positionof the ZFC susceptibility is estimated to be 170 K. Thevalue of Tmax is much smaller than bifurcation tempera-ture (240 K) signaled by splitting between the ZFC andthe FC curves. A similar trend is also seen for NiFe2O4

nanoparticles.In addition to superparamaginetic blocking, the ZFC

curve for the NiFe2O4 hollow sphere sample andnanoparticle sample has broad peaks at Tf ≈13 K and 10K, respectively, The FC branch exhibits a steep inceaseat Tf . However, the memory properties corresponding toa super-spin-glass are not observed at the temperaturesnear Tf [8]. This suggests that the anomaly in the ZFCand the FC susceptibilities at Tf is caused by a freez-ing of the surface spin [14, 15]. To confirm the surface

Fig. 4. (a) Field dependence of δTf = 1− Tf (H)

Tfof NiFe2O4

nanoparticles (◦) and hollow spheres (•). (b) ZFC suscepti-bility of NiFe2O4 hollow spheres around Tf .

spin freezing in nanoparticles, we investigated the fielddependence of Tf under fields H from 10 to 2000 Oe.The value of Tf is observed to decrease with increasingfields as in Fig. 4(b). Figure 4(a) shows the change ofδTf = 1 − Tf (H)

Tffor NiFe2O4 nanoparticels and hollow

sphere samples. In case of nanoparticles, the reducedtemperature δTf is fitted to a power law,

δTf = 1 − Tf (H)Tf

∝ H23 ,

as demonstrated by the so-called de Almeida–Thouless(AT) line, which is a well-known signature of Ising spin-glass behavior [15,16]. Such a field dependence of the Tf

is in good agreement with the result previously reportedby Tackett et al. [17]. Thus, the anomaly at Tf denotes afreezing of the surface spins that surround the nanopar-ticle’s ferrimagnetic core. In the case of a hollow sphere,however, the field dependence of Tf cannot be repro-duced by using the AT line, or the Gabey–Touloose (GT)line for a randomly-anisotropic 3D Heisenberg spin-glass;δTf ∝ H2 [18]. The observed field dependence of Tf can-not be fitted by using simple power laws. This suggeststhat the transition is subject to the influence of inter-particle interactions, e.g., exchange coupling and dipole-diplole interactions, as well as surface spin freezing.

IV. CONCLUSIONS

In conclusion, we observed a superparamagnetic-typeblocking process in sub-micron-sized hollow spheres as-sembled from MFe2O4 (M= Ni, Mg) nanoparticles pre-pared by using the template method with a wet chemi-cal process. Furthermore, a spin-glass-like behavior dueto surface spin freezing is found in Ni-ferrite nanoparti-cles. The surface spin freezing temperatures Tf are muchlower than the blocking temperature, and the magneticfield dependence is different between the nanoparticlesand the fabricated hollow spheres. The spin-glass na-ture of the Ni-ferrite nanoparticle is demonstrated bythe magnetice field dependence of Tf following the well

Structual and Magnetic Properties of MFe2O4· · · – Kensuke Konishi et al. -675-

known Almeida-Thouless line (δTf ∝ H23 ). In contrast,

the Tf (H) of the Ni-ferrite hollow sphere hints at thefailure of the Gabay-Toulouse theory as well as the ATtheory. This results suggest that the transition is subjectto the influence of the inter-particle interactions in thehollow sphere nanomagnet.

REFERENCES

[1] K. Raj and R. J. Boulton, J. Magn. Magn. Mater. 8, 233(1987).

[2] A. Moser, K. Takano, D. T. Margulies, M. Albrecht, Y.Sonobe, Y. Ikeda, S. Sun and E. E. Fullerton, J. Phys.D: Appl. Phys. 35, R157 (2002).

[3] M. D. Mukadam, S. M. Yusuf, P. Sharma and S. K.Kulshreshtha, J. Magn. Magn. Mater. 272–276, 1401(2004).

[4] J. L. Dormann, R. Cherkaoui, L. Spinu, M. Nogues, F.Lucari, F. D’Orazio, D. Fiorani, A. Garcia, E. Tronc andJ. P. Jolivet, J. Magn. Magn. Mater. 187, L139 (1998).

[5] D. Fiorani, J. L. Dormann, R. Cherkaoui, E. Tronc, F.Lucari, F. D’Orazio, L. Spinu, M. Nogues, A. Garciaand A. M. Testa, J. Magn. Magn. Mater. 196–197, 143(1999).

[6] T. Jonsson, J. Mattsson, C. Djurberg, F. A. Khan, P.Nordblad and P. Svedlindh, Phys. Rev. Lett. 75, 4138(1995).

[7] S. Sahoo, O. Petracic, Ch. Binek, W. Kleemann, J. B.Sousa, S. Cardoso and P. P. Freitas, Phys. Rev. B 65,134406 (2002).

[8] K. C. Zhang and B. G. Liu, Phys. Lett. A 373, 2760(2009).

[9] J. Yu and X. Yu, Environ. Sci. Thechnol. 42, 4902(2008).

[10] G. Duan, W. Cai, Y. Luo and F. Sun, Adv. Funct. Mater.17, 644 (2007).

[11] Y. Zhang, Z. Huang, F. Tang and J. Ren, Thin SolidFilms 515, 2555 (2006).

[12] H. Shiho and N. Kawahashi, J. Colloid Interface Sci. 226,91 (2000).

[13] H. Yoshikawa, K. Hayashida, Y. Kozuka, A. Horiguchiand K. Awaga, Appl. Phys. Lett. 85, 5287 (2004).

[14] R. H. Kodama, A. E. Berkowitz, E. J. McNiff, Jr. and S.Foner, Phys. Rev. Lett. 77, 394 (1996).

[15] B. Martınez, X. Obradors, L. l. Balcells, A. Rouanet andC. Monty, Phys. Rev. Lett. 80, 181 (1998).

[16] J. R. L de Almeida and D. J. Thouless, J. Phys. A: Math.Gen. 11, 983 (1978).

[17] R. J. Tackett, A. W. Bhuiya and C. E. Botez, Nanotech-nology 20, 445705 (2009).

[18] M. Gabay and G. Toulouse, Phys. Rev. Lett. 47, 201(1981).